Polarity insensitive solid state switch

ABSTRACT

A polarity insensitive solid state switch controlling a shunt load, wherein the switch includes first and second transistors connected in anti-parallel in series with the shunt load; and a longitudinal current sensor controlling the control electrodes of both transistors. The current sensor may be a current sensing resistor in series with a second load. Each transistor may be a bipolar transistor and the current sensing resistor may be connected between the base and emitter of each transistor.

TECHNICAL FIELD

This invention relates to a filter circuit, and will be described withreference to an ADSL/POTS filter.

BACKGROUND ART

ADSL transmission systems require low pass filters at the customer endto prevent high frequency ADSL signals from interfering with the POTSservice and also to prevent HF transients from the POTS interfering withthe ADSL transmission.

In the context of a telephone line carrying both ADSL & POTS signals, ithas been found that multiple in-line filters on a single line causeprogressive degradation of return loss at the POTS/Line ports.

Australian Application No. 36813/99 discloses the use of a switchedcapacitor to change the filter characteristics when the phone goesoff-hook and to reduce the effect of multiple filters on the one line.This may, for example change the filter from a second order filter to afourth order filter.

DISCLOSURE OF THE INVENTION

According to the present invention we propose to use a switchedimpedance inserted on the low frequency side of the filter to change thefilter characteristics when the customer equipment (such as a phone)goes off-hook.

In a further embodiment we propose the use of a complex impedance ratherthan a capacitor as the switched impedance. In this embodiment both thefilter rolloff characteristics and telephone sidetone performance areimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an arrangement in which the filter characteristics arealtered by inserting a switched capacitance as an element of the filter.

FIG. 2 shows a circuit embodying the invention.

FIG. 3 shows an embodiment of a return loss correction circuit for usein the circuit of FIG. 2.

FIG. 4 shows an embodiment of the invention employing an innovativeswitching arrangement.

FIG. 1 shows a filter arrangement incorporating a switchable capacitor,8, within the filter.

The filter includes the inductors 2, 3, 5 and 6 and the capacitor 7 aswell as the switchable capacitor 8 connected between the junction ofinductors 5 and 6 and via switch 9, the junction of inductors 2 and 3.

When switch 9 is open and capacitor 8 is disconnected from the filter,the filter has a second order lowpass characteristic. Closing the switch9 incorporates the capacitor 8 into the filter and substantially altersthe filter performance. The filter then has a fourth ordercharacteristic with faster roll-off rate and the roll-off point is movedto a substantially higher frequency, so that the filter passes more ofthe higher frequency bond.

In a known embodiment, described in our application No. 36/813,99, theswitch 9 is a metallic switch operated by an electromagnetic relay coil(not shown) incorporated in the telephone loop. Thus, when the phonegoes OFF-HOOK, the relay is energized and closes the switch 9.

According to an embodiment of our present invention, we do not modifythe order of the filter by using a switchable capacitor as an element ofthe filter.

Preferably, we provide a return loss correction function activated whenthe customer equipment goes off-hook by incorporating a switchableimpedance on the low frequency side of the filter. Because we do notalter the filter characteristics, we reduce the consequences of thechange in filter characteristics.

As shown in the embodiment of FIG. 2, we incorporate a switchable returnloss correction circuit including impedance 220 between the customerequipment port and a fixed return loss correction circuit 1, 4, 10, 11,12, 13, the fixed return loss circuit providing correction over a firstrange, for example, 2 to 4 kHz. The switchable return loss correctioncircuit 220 by closing switch 209 provides correction over a widerrange, e.g. 3 to 20 kHz. The fixed return loss circuit is balancedbetween the two wires of the line.

We have found that when the filter is to match a complex impedance thereturn loss characteristics can be improved by using a complex impedanceto more nearly approximate the reference impedance than is possible withthe use of a capacitor alone.

As shown in FIG. 3, the switchable return loss correction circuitincludes a capacitor 221, an inductor 222, and a resistor 223. Thesecomponents are chosen to optimize return loss when the line is looped.

The fixed return loss correction circuit is automatically connectedacross the line by line sensing means such as a relay which is activatedby the line current when the line is looped.

In a further inventive embodiment we use a novel solid state switchinstead of an electro-mechanical relay.

According to the first embodiment of the switch there is provided apolarity insensitive solid state switch controlling a shunt load, theswitch including:

-   -   first and second transistors connected in anti-parallel in        series with the shunt load; and    -   a longitudinal current sensor controlling the control electrodes        of both transistors.

Preferably, the current sensor is a current sensing resistor in serieswith a second load. Each transistor may be a bipolar transistor and thecurrent sensing resistor is preferably connected between the base andemitter of each transistor.

A circuit embodying this which is shown in FIG. 4. A pair of transistors41, 42 are connected in anti-parallel in series with a return losscorrection circuit including capacitor 46, inductor 47 and resistor 48.A load sensing resistor 43 is connected between the base and emitter ofeach of the transistors 41, 42. The transistors 41 and 42 are arrangedso that they are both “OFF” when the line is not looped (no linecurrent), and so that one or other of the transistors is “ON” when theline is looped (line current present), the voltage drop across resistor43 switching one of the transistor “ON” depending on the line polarityas shown resistor 43 is connected from the emitter to the base oftransistor 41, and from the base to emitter of transistor 42 assuming,e.g., that the upper line is positive with reference to the lower line.Thus the base-emitter voltage applied to transistor 41 by resistor 43 isin the opposite sense to that applied to transistor 42.

As shown in FIG. 4 the switchable return loss correction circuit 46, 47,48 is connected on the low frequency side of filter 412, 413, 414. Inthis embodiment, another return loss correction circuit, 409, 410, 411,415, 416, 417 is connected between the switchable return loss correctioncircuit and the filter.

1. A polarity insensitive solid state switch controlling a shunt load,the switch comprising: first and second transistors connected inanti-parallel with each other and in series with the shunt load; and asingle current sensor connected in series with the shunt load andcontrolling control electrodes of the first and second transistors.
 2. Aswitch as claimed in claim 1 wherein the current sensor comprises acurrent sensing resistor connected in series with the shunt load.
 3. Aswitch as claimed in claim 1 wherein each transistor is a bipolartransistor and the current sensor is connected between a base and anemitter of each bipolar transistor.
 4. A switch as claimed in claim 1wherein each transistor is an NPN transistor.